1. Field of the Invention
This invention relates generally to a method and system for controlling the temperature of superheated steam generated within a boiler, and more particularly to method and system for controlling boiler-superheated steam temperature in the process of starting up the boiler attendant to turbine start-up in a power plant provided with a turbine bypass valve.
2. Description of the Prior Art
Some power plants are provided with a turbine bypass valve in parallel with a main steam stop valve and a turbine so as to cause superheated steam generated within a boiler to be led directly into a condenser. The turbine bypass valve serves to bypass surplus superheated steam so as not to lead it into the turbine to protect the turbine. Assuming that an electric generator in a power plant is interrupted upon occurrence of load interruption in the power transmission system, the power plant becomes unable to supply power to the transmission system, so that turbine no longer needs abundant superheated steam. Should the same amount of flow of superheated steam as in a rated load operation be led into the turbine even after interruption from the transmission system, the turbine will unnecessarily undergo accelerated rotation which may result in mechanical damage. Thus, in such event, the turbine bypass valve is opened so that surplus superheated steam may be led directly into the condenser so as to protect the turbine. Naturally, when the electric generator is interrupted from the power transmission system, a control will be made such that the flow rate of the superheated steam generated within the boiler is suppressed. However, the response characteristics of such control is extremely slow, so that as described above, the power plant is provided with the turbine bypass valve.
As seen above, the turbine bypass valve per se is intended only for such emergency use as the above-described load interruption in the power transmission system, so that it essentially does not participate in the temperature control of superheated steam in the process of starting up the boiler attendant to turbine start-up. Therefore, even when a power plant is provided with the turbine bypass valve, there is no difference between a power plant without the turbine bypass valve in terms of the temperature control of superheated steam in the process of starting up a boiler attendant to turbine start-up, and such temperature control has been carried out so far in a manner as nextly described.
In general, the temperature control of superheated steam in the region of lower boiler load, i.e. in the process of boiler starting up is said to be complicated and difficult due to its non-linear process gain characteristics. Also it is generally known that the response of the superheated steam with respect to changes of combustion gas temperature or fuel flow rate at the boiler furnace outlet is slow, so that the response characteristics of the control system is inferior. Thus, it takes a substantially lengthy time to obtain a superheated steam condition (e.g. superheated steam temperature meeting certain specified value) required at the instant of turbine start-up (leading steam into turbine). In addition, the superheated steam condition required when leading steam into the turbine varies depending upon a "mode" of the individual power plant so that the control of superheated steam should be made so as to conform to the individual "mode". For instance, in the case of a "hot mode" in which the boiler and turbine have higher remaining heat, the superheated steam condition should be in a higher temperature mode, and to the contrary, in the case of a "cold mode", the superheated steam condition should be in a lower temperature mode, otherwise it may result in occurrence of thermal fatigue on the turbine inner metal. Such modes are generally determined on the basis of process quantities indicative of the turbine start-up mode, such as the inner metal temperature of the first stage steam chamber of the turbine. As described above, the superheated steam condition required at the instant of leading steam into the turbine varies depending upon a mode of individual power plant, so that the temperature control of superheated steam is generally said to be complicated and difficult.
For controlling such superheated steam temperature, there is provided a boiler controlling apparatus, which includes two temperature control functions, including a first temperature control function that raises temperature and pressure of fluid at the water-cooling wall outlet of the boiler, and a second temperature control function that maintains the superheated steam temperature of the boiler outlet at a temperature capable of leading the steam into the turbine. The second temperature control function maintains the superheated steam temperature of the boiler outlet at a temperature capable of leading the steam into the turbine by controlling a combustion gas temperature at the boiler furnace outlet. Namely, after the boiler has started up, the boiler controlling apparatus firstly controls, by the function of the first temperature control function, to raise the temperature and pressure of the fluid at the water-cooling wall and to obtain superheated steam. When superheated steam is obtained, the second temperature control function then controls a combustion gas temperature at the boiler furnace outlet so as to cause the superheated steam temperature at the boiler outlet to become a temperature capable of leading steam into the turbine. This obtains the superheated steam in conformance with the mode of the power plant, and enables the turbine to safely start up.
Now, during the execution of such control, the turbine is not started up, namely prior to leading the steam into the turbine, so that the main steam stop valve is usually closed, thus the flow rate of the superheated steam is scarcely increased. Namely, the superheated steam can hardly flow, so that temperature rises of the superheated steam at the boiler outlet and the turbine inlet are extremely slow. Thus, it requires a longer time for the superheated steam temperature at the boiler outlet to reach the temperature capable of leading the steam into the turbine. This prevents the turbine from rapidly starting up and inevitably limits the turbine response to a certain extent. That is, this has become an obstacle in terms of carrying out a peak load power generation in which a quick power plant start-up response is required. Should the superheated steam be led into the turbine in an attempt at quicker power plant start-up in spite of the fact that the superheated steam temperature has not reached the temperature capable of leading the steam into the turbine, there results in occurrence of thermal fatigue on the inner metal of the turbine which adversely affects the turbine as such, so that such operation is not preferable for the sake of security.
As described above, the flow of superheated steam is scarcely present in the process of boiler starting up attendant to turbine start-up in the previous method of control, so that a considerably longer time has been required to obtain the necessary condition for superheated steam to be led into the turbine.